Chemical effects during the formation of various types of femtosecond laser-generated surface structures on titanium alloy

Abstract

In this contribution, chemical, structural, and mechanical alterations in various types of femtosecond laser-generated surface structures, i.e., laser-induced periodic surface structures (LIPSS, ripples), Grooves, and Spikes on titanium alloy, are characterized by various surface analytical techniques, including X-ray diffraction and glow-discharge optical emission spectroscopy. The formation of oxide layers of the different laser-based structures inherently influences the friction and wear performance as demonstrated in oil-lubricated reciprocating sliding tribological tests (RSTTs) along with subsequent elemental mapping by energy-dispersive X-ray analysis. It is revealed that the fs-laser scan processing (790 nm, 30 fs, 1 kHz) of near-wavelength-sized LIPSS leads to the formation of a graded oxide layer extending a few hundreds of nanometers into depth, consisting mainly of amorphous oxides. Other superficial fs-laser-generated structures such as periodic Grooves and irregular Spikes produced at higher fluences and effective number of pulses per unit area present even thicker graded oxide layers that are also suitable for friction reduction and wear resistance. Ultimately, these femtosecond laser-induced nanostructured surface layers efficiently prevent a direct metal-to-metal contact in the RSTT and may act as an anchor layer for specific wear-reducing additives contained in the used engine oil.

Repellent rings at titanium cylinders against overgrowth by fibroblasts

“…The brownish color of the regions (3) in Figure 6 can be attributed to oxide-layer thickness in the order of 100 nm [15,16]. The effective thickness of the oxide-layer in femtosecondlaser and anodized area (4) is probably considerably larger [17], considering that the irradiation process takes place in air environment with a combination of (i) high energy deposited by the laser that increases strongly the sample surface temperature and (ii) an increase of the effective surface induced by the topography inherent of the spike structures. The latter effect is relevant as well for the laserinduced oxidation as for the electrochemical oxidation.…”

Ten Open Questions about Laser-Induced Periodic Surface Structures

“…One of the main difficulties of practical applications of LIPSS arises from the fact that the desired surface functionality is usually affected not only by the grating-like surface topography but also by its specific surface chemistry. The latter includes laser-induced chemical alterations, e.g., superficial oxidation [16,[55][56][57] caused by laser-processing in an air environment but also from redeposited material (debris) [58] or from post-irradiation adsorption of molecules such as hydrocarbons during sample handling and ambient storage [58][59][60][61]. Thus, there has been a long-lasting debate in the LIPSS-community regarding the relevance of the topography vs. the interfacial chemistry for surface functionalization.…”

Quo Vadis LIPSS?—Recent and Future Trends on Laser-Induced Periodic Surface Structures

“…Depth-resolved surface analytical studies on LIPSS based on Auger electron microscopy (AEM) [ 9 ], time-of-flight secondary ion mass spectrometry (TOF-SIMS) [ 40 ], and glow-discharge optical emission spectroscopy (GD-OES) [ 41 ] indicate a graded oxidized surface layer of several hundreds of nanometer thickness. Such nanostructured oxide layers may, however, exhibit beneficial tribological effects, particularly in combination with “anti-wear” additives, such as zinc-dialkyl-dithiophosphate (ZDDP), contained in lubricants, such as commercial engine oil [ 41 , 42 , 43 ].…”